Africa, where birth rates were expected to decline faster than they have, accounts for most of the increase.

The population of Africa is about 1.1 billion. It is now predicted to expand fourfold to 4.2 billion by 2100.

In other areas of the world, fewer major changes are forecast. Declining fertility in Europe may lead to a small drop in population, while other regions see modest increases due to longer life expectancy.

China's population is expected to show the biggest decrease, falling from 1.4 billion today to 1.1 billion by the end of the century.

Lead researcher Professor Adrian Raftery, from the University of Washington in the US, said: "The fertility decline in Africa has slowed down or stalled to a larger extent than we previously predicted and, as a result, the African population will go up.

"These new findings show that we need to renew policies, such as increasing access to family planning and expanding education for girls, to address rapid population growth in Africa."

The findings are based on a United Nations report which broadly estimates that the world's population will range between seven and nearly 17 billion by 2100. The Washington team used advanced forecasting methods to fine tune the UN statistics.

Global population passed six billion in 1999 and reached seven billion in 2011.